Epithelial ovarian cancer risk is reduced among women who have been one or more full-term pregnancies, and among women who have used oral contraceptives (OC's). Further, risk decreases with increasing duration of OC use. Apart from these observations however, little is known about the etiology of the disease. We propose a combined analysis of all US case-control studies of ovarian cancer to test major hypotheses. These include: i) risk for epithlial ovarian cancer increases with postmenopausal exogeneous estrogen use, with moderate or severe obesity, with increasing estimated years of ovulation: ii) among the parous, risk decreases with increasing number of full-term pregnancies. Age at first birth has no effect on risk, after adjusting for number of full-term pregnancies; iii) the protective effects of OC use are independent of age at first use, time since first use, parity, menopausal status, age at observation, specific formulation, subtype of tumor, iv) pregnancy; v) risk for epithelial ovarian cancer is unrelated to history of surgical sterilization and consumption of coffee, tobacco and alcohol. The combined analysis also will evaluate for the first time the effects of epithelial ovarian cancer risk factors on more than 100 nonepithelial ovarian cancers and more than 200 borderline tumors. Finally, the combined data base of more than 2000 cases of epithelial ovarian cancer risk factors. Among the potential benefits of such analysis are: new findings inaccessible to any one data set; new hypotheses about causal mechanisms for ovarian cancer that have been suggested by discussions among national experts; and new statistical methods for combining data sets for rare chronic diseases.